This is a ctypes based python wrapper for snap7. Snap7 is an open source, 32/64 bit, multi-platform Ethernet communication suite for interfacing natively with Siemens S7 PLCs. python-snap7 support ...

And "-55" of course throws the ValueError. When I run the same query with snap7 ( cpp or plain-c ), I get the correct values on windows and linux. Therefore I guess ...